Adaptive broadband post-distortion receiver for digital radio communication system

ABSTRACT

An adaptive broadband post-distortion receiver for wireless digital communications improves performance of a wireless digital communications system. The improved performance of the system includes improved linear AM-AM and AM-PM responses approaching saturation. Indeed, the proposed adaptive broadband post-distortion receiver has an effect on attributes such as bit error rate and transmitter power efficiency and, in turn, on modulation and bandwidth. The adaptive broadband post-distortion receiver is configured with an adaptively controlled IF post-distorter located at the IF stage of the receiver. The placement of the distortion canceling function in the IF section of the receiver lends itself to broadband applications. The post-distorter is configured to cancels the distortion produced by the transmitter and receiver as it is adaptively controlled using bit error rate calculations. The distortion canceling utilizes bit error rate information that is otherwise available in the receiver.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to digital communication systemsand, specifically, to wireless digital communication systems designedfor improved performance such as power efficiency and bit error rate.

[0003] 2. Background Overview

[0004] Wireless systems include satellite, cellular, fixed access,wireless LANs (local area networks) and personal AN (area networks). Thetrend in wireless systems involves integration of the various networks,increased data rates, and proliferation of services such as Internet,data and image transmission or downloads, and voice over IP (InternetProtocol). Thus, to accommodate this trend performance attributesaccounted for in wireless communications systems include sensitivity,selectivity, dynamic range, data rate capacity, power efficiency, andbit error rates. In addition, manufacturing and marketing attributessuch as low cost, high reliability, and flexibility are becomingincreasingly important.

[0005] Digital wireless communication systems include a transmitter, areceiver (including a software-defined radio) or both (a combinationthat is referred to as “transceiver”). The block diagram of FIG. 1,shows typical transmitter and receiver components in a digital wirelesscommunication system 10. In such a system there are a number of possiblelocations where distortion is a factor, including the baseband stage 11& 12 (with digital data or low frequency such as <30 MHz), intermediatefrequency stage 13 & 14 (IF, such as 2 GHz), and radio frequency stage15, 16 & 17 (RF, such as 6 GHz) including mmW (millimeter wavefrequencies, such as 38 GHz).

[0006] In digital communication systems, the enabling technology is acombination of software control, RF and IF circuitry, and digitalcircuitry, including signal-processing components. The design of digitalcommunication systems presents a challenge of optimizing the RF circuitfunctionality to address one or more of the foregoing performance,manufacturing, and marketing attributes and the complexities theyintroduce.

[0007] For example, in the transmitter, the RF power amplifier isexpected to meet peak and average power specifications and provide highpower efficiency within the specified frequency range. However,distortion in amplifiers generates AM-AM and AM-PM non-linearities (AMstands for amplitude modulation, PM stands for phase modulation). Hence,communication systems using linear modulation techniques such asquadrature amplitude modulation (QAM) are restricted by the performanceof the transmitter and receiver. One of the restrictions is theaforementioned nonlinear characteristic of the power amplifier thatcauses the AM-AM and AM-PM distortion.

[0008] There are a number of approaches for controlling linearity anddistortion levels. The simplest approach involves using higher powerdevices in the power amplifier while operating at a high back-off ratioin the output power level. The drawbacks of this approach includeincreased DC power consumption, higher cost, and lower reliability.

[0009] A common approach for improving power amplifier linearity is touse RF signal feedback. In a higher frequency range, the tradeoff forimproved linearity is reduced gain and, in turn, reduced power level. Intransmitters, the reduction in output power level has an adverse effecton the allowable distance between the transmitters and correspondingreceivers.

[0010] Baseband signal feedback—which is analogous to pre-distortion ofpower amplifier input—is used in transmitters to provide for some of thedeficiencies of RF signal feedback. This approach involves basebandsignal modulation of an RF carrier, and amplification of the modulatedRF carrier signal by the non-linear RF power amplifier. A sample of theamplified, modulated carrier signal is demodulated and fed back to theinput of a baseband amplifier where it is combined with the basedbandinput of that amplifier. The introduction of the demodulated sampledcarrier signal at the input of the baseband amplifier creates apre-distortion of the baseband signal to counteract the distortion fromthe RF power amplifier's non-linearity. The tradeoff in this case is afeedback loop delay that limits the possible bandwidth of transmittedsignals.

[0011] RF signal pre-distortion is another approach. The objective inthis approach is to directly cancel the distortion of the poweramplifier by pre-distorting the signal going into it. As in the case ofthe baseband pre-distortion scheme, the RF signal pre-distortion can beadaptive using a cancellation scheme based on the transmitter signal.

[0012] Namely, some transmitters use a cancellation scheme as avariation of the pre-distortion approach. This involves adaptivefeedback where the gain of cancellation amplifiers is adaptivelymodified. Adaptive control methods are used to adjust a distortioncanceling circuitry for changing conditions such as transmitter powerlevels, temperature, or aging. Examples of methods for adaptive controlinclude conversion to baseband, conversion to IF, and predictivecalibration.

[0013] In the conversion to baseband scheme, a simplified receiver(located in the transmitter) is used to sample the modulated transmitsignal. The distortion canceling circuit is adjusted based on thedemodulated baseband signal. The gain adjustment combined with thepre-distortion reduces amplitude and phase distortions. However, thisapproach assumes that the distortion is a relatively small component ofthe signal. Moreover, frequency changes would negate the correctiveeffects of the cancellation scheme.

[0014] In the conversion to IF scheme, the modulated output signal(usually from the transmitter power amplifier) is down converted to anIF signal. The modulated IF signal is filtered to monitor the amount ofdistortion. Then a distortion canceling circuit is adjusted to minimizethe distortion. This approach suffers from deficiencies similar to thoseoutlined above.

[0015] With predictive calibration, the transmitter uses a look-up tablebased on temperature and transmitter power level to adjust thedistortion canceling circuit. This is not a true adaptive method, but anopen loop technique requiring careful characterization of thetransmitter.

[0016] Over time, numerous combinations of distortion canceling andadaptive control methods have been broached. Commonly, these techniqueshave been employed in the transmitter sections of wireless digitalcommunications systems.

[0017] However, given that distortion remains a factor in digitaltransmission, design considerations of dynamic range, bit error rate,power efficiency, data rate capacity and the like also remain.Accordingly, in dealing with the associated design challenges a betterapproach is needed.

SUMMARY OF THE INVENTION

[0018] The present invention provides an adaptive broadbandpost-distortion receiver for wireless communications. This approachimproves the performance of a wireless digital communications systemwith a combination transmitter and receiver by improving linear AM-AMand AM-PM responses approaching saturation. Indeed, the proposedadaptive broadband post-distortion receiver has an effect on attributessuch as bit error rate and transmitter power efficiency and, in turn, onmodulation and bandwidth. In this receiver, an adaptively controlled IFpost-distorter compensates for the non-linear distortion in bothtransmitter and receiver sections of a wireless digital communicationsystem. The IF post-distorter is configured to cancels the distortionproduced by the transmitter and receiver by being adaptively controlledvia performance monitoring with bit error rate calculations orsignal-to-noise ratios. This approach avoids introducing limitationsinto and allows maintaining the transmitter spectrum response. Moreover,the IF post-distorter is adjustable to fit a variety of transmitters andreceivers.

[0019] In fashioning the adaptively controlled IF post-distorter, adistortion canceling circuit is employed in the IF section of thebroadband receiver. The distortion canceling circuit utilizes bit errorrate information that is inherently available in the receiver. The biterror rate information is derived from available communication systemperformance values without requiring additional circuitry. The bit errorrate information is easily retrievable from the receiver for processingby a microprocessor, and it is used to adaptively adjust the distortioncanceling circuit for improved performance.

[0020] The placement of the distortion canceling circuit in the IFsection of the receiver lends itself to broadband applications. The RFtransmit and receive frequencies of the wireless communication systemcan be changed independently of the IF frequency. Thus, a single designis suitable for work at various RF frequencies, including frequenciesfrom 2 GHz to greater than 40 GHz.

[0021] It is further envisioned that the IF post-distorter will becapable of independently adapting to the non-linear characteristics ofvarious power amplifiers or receivers. The non-linear characteristics ofthe power amplifiers or receivers do not need to be known in advance ofthe cancellation process.

[0022] To recap, in accordance with a purpose of the invention areceiver system for distortion compensation is envisioned to include anIF post-distorter in the IF section of the receiver, a bit error ratesource, and a controller. The controller is configured to obtainresidual bit error rate (RBER) from the bit error rate source, and touse the RBER in performing an optimization process for adjusting thepost-distorter to reduce the RBER. The controller is fashioned as adigital circuit including a microprocessor. In such a system, thepost-distorter includes a power splitter, a delay line setting one pathfrom the power splitter, a cuber generator setting a second path fromthe power splitter, a vector modulator connected to the cuber generatoralong the second path, and a power combiner at which the first andsecond paths are joined. The receiver in which such system is embodiedis communicatively connected to a transmitter via an antenna. Hence, thepost-distorter is envisioned to affect reduction in the RBER regardlessof variations in transmitter power levels, or temperature and aging ofthe transmitter and receiver.

[0023] In further accordance with a purpose of the invention a methodfor adaptive broadband post-distortion is performed in a receiver systemsuch as the foregoing. Generally, a method for adaptive broadbandpost-distortion includes receiving an RF signal from a transmitter, downconverting the RF signal into an IF signal, and processing the IFsignal. As implemented in one instance, the method further includesintroducing post-distortion into the processed IF signal, demodulatingthe post-distorted, processed IF signal to create a baseband signal,processing the baseband signal, retrieving a bit error rate associatedwith the processed baseband signal, and performing an optimizationprocess according to which the post-distortion is adjusted. In thisinstance, the optimization process is a random or gradient optimizationprocess.

[0024] Advantages of the invention can be understood by those skilled inthe art, in part, from the description that follows. Advantages of theinvention can be realized and attained from practice of the inventiondisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate several embodimentsof the invention and together with the description, serve to explain theprinciples of the invention. Wherever convenient, the same referencenumbers will be used throughout the drawings to refer to the same orlike elements.

[0026]FIG. 1 illustrates transmitter and receiver components in atypical digital wireless communication system.

[0027]FIG. 2 illustrates a wireless communications system block diagramshowing an IF post-distorter in the receiver. The bit error rateperformance monitor utilizes a microprocessor to adjust thepost-distorter for improved bit error rate performance.

[0028]FIG. 3 is a block diagram illustrating the IF post-distorter inmore detail.

[0029]FIG. 4 is a block diagram illustrating a cuber generator that ispart of the IF post-distorter.

[0030]FIG. 5 is a block diagram illustrating a squarer generator that ispart of the cuber generator.

[0031]FIG. 6 is a block diagram illustrating a vector modulator that ispart of the IF post-distorter.

[0032]FIG. 6a is a flow diagram illustrating control of the IFpost-distorter using bit error rate information.

[0033]FIG. 7 is a graph illustrating a summing vector diagram of thevector modulator.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention is based, in part, on the observation thatpost-distortion techniques have not been a factor considered in thedesign of receivers in digital communication systems. To achieveimproved performance, the present invention introduces post-distortiontechniques into the design of broadband receivers, as compared with thevarious pre-distortion techniques used in transmitters. As a functionaland architectural strategy, the invention contemplates an adaptivebroadband post-distortion receiver for wireless communications which hasan adaptively controlled IF post-distorter. The adaptively controlled IFpost-distorter compensates for the non-linear distortion in bothtransmitter and receiver sections of the wireless digital communicationsystem.

[0035] To enable one of ordinary skill in the art to make and use theinvention, the description of the invention is presented herein in thecontext of a patent application and its requirements. Although theinvention will be described in accordance with the shown embodiments,one of ordinary skill in the art will readily recognize that there couldbe variations to the embodiments and those variations would be withinthe scope and spirit of the invention.

[0036] As mentioned, the present invention operates in the context ofwireless (radio) digital communications and is embodied, for example, ina wireless digital communications system as shown in FIG. 2. The upperportion of the diagram illustrates a typical heterodyne transmitter in amicrowave radio digital communications system 100 using QAM (quadratureamplitude modulation).

[0037] For data input signals, the baseband (BB) processing stage 101provides a number of required signal processing operations to conditionthe signal to be carried by the radio system 100. Then the quadratureamplitude modulator 102 modulates carriers I (amplitude) and Q (phase)to yield a QAM signal at the intermediate frequency (IF). Assuming thatthe I and Q carriers are m_(I) and m_(Q), respectively, and that thelocal oscillator (LO) frequency is expressed in the form of w_(c), themodulated (QAM) IF signal, S_(IF), is expressed as a function of time,t, as follows: S_(IF)(t)=m_(I) cos(w_(c)t)−m_(Q) sin(w_(c)t). The valuesm_(I) and m_(Q), are the cartesian representations of the amplitude andphase of the IF signal.

[0038] The IF processing stage 103 provides IF filtering, IFamplification, and cable compensation to overcome the loss of signal dueto cable losses between the modulator 102 and power amplifier 106. TheIF/RF converter stage 104 converts the modulated IF signal to an RFsignal. Then the RF processing stage 105 provides RF amplification todrive up the RF power amplifier.

[0039] The RF power amplifier stage 106 raises the power level of the RFsignal to the required transmitter signal at the antenna. As suggestedbefore, a RF power amplifier has a non-linear transfer characteristicmanifested in terms of a dominant third-order non-linear behavior. Thisnon-linear characteristic causes amplitude-to-amplitude modulation(AM-AM) and amplitude-to-phase modulation (AM-PM) distortions, as wellas spectrum spreading. Spectrum spreading causes interference to signalsin any adjacent channels and creates impairments that adversely affectcommunications system performance attributes such as the residual biterror ratio (RBER).

[0040] The filter stage 107 reduces the levels of out-of-channelspurious emissions, and the levels of the continuous spectrum generatedin the transmitter by the non-linear RF power amplifier. Finally, thetransmit Antenna 108 directs the transmit signal to the receiver antenna109 in a point-to-point or point-to-multipoint link using free space asthe propagation medium.

[0041] The lower portion of FIG. 2 illustrates the heterodyne receiver100B with the addition of the IF post-distorter 115 (or simplypost-distorter). The receiver antenna 109 has a preferred highlydirectional transmitter signal reception pattern in order to achievehigh antenna gain and interference reduction. The receiver filter stage110 is configured to block the near-end transmitter signal from reachingthe receiver, block the low noise amplifier (LNA, 111) noise at theimage frequency, and control the spurious receiver responses. The LNA111 provides amplification of the received filtered signals and sets thereceiver noise figure. The receiver RF processing stage 112 provides anadditional filter to block the receiver amplifier noise at the mixerimage frequency. This processing stage further provides a portion ofautomatic gain control (AGC) circuit (not shown) to compensate for thevarying receiver signal levels at the receiver antenna 109. Then theRF/IF converter 113 converts the receiver RF signal to a receiver IFsignal. The receiver IF processing 114 provides IF amplification, andanother portion of the AGC circuit to hold the signal being supplied tothe post-distorter at a constant level.

[0042] The post-distorter 115 generates distortion signals to compensatefor the amplitude and phase distortions of the transmit RF poweramplifier 106 and the receiver. More details of the post-distorter 115will be provided below.

[0043] The demodulator 116 shifts the receiver IF signal to basebandsignal using a carrier on the receiver signal. Then the receiverBaseband (BB) Processing 117 provides a number of required signalprocessing operations complementary to those of the transmitter BBprocessing stage (101) such as decision circuitry (A/D converter),decoding, descrambling, error detection, demultiplexing, timingrecovery, and adaptive equalization. The controller 118, typically amicroprocessor, with its associated logic circuitry provides processingmanagement functions such as interface, control, alarm, monitoring,switching, and telemetry.

[0044] As noted above, conventional techniques employ pre-distorters intransmitters. Apparently, when a pre-distorter is adjusted to improvetransmit spectrum, such adjustment may not necessarily improve theresidual bit error rate (RBER) in any significant way. In an adaptivecontrol, the adjustment to improve the RBER of the communications systemmay in fact produce a worse transmit spectrum, especially in broadbandapplications (wide signal bandwidth applications). Besides, in someimplementations that use pre-distortion there is no feedback in thecommunications system. It means that such systems are implemented as anopen loop in which, to be cancelled, the distortion must be known inadvance and it requires calibration.

[0045] Accordingly, an advantage of using the post-distorter in thereceiver is that the adaptive control maintains the transmit spectrumunabated while improving the RBER significantly. This approachrepresents a closed-loop system and thus requires no advanced knowledgeof the distortion to be cancelled.

[0046] The RBER information is inherently available in the receiver(from the bit error rate performance monitor 120) and to be obtained itdoes not require use of additional circuitry. This information is usedto adaptively adjust the distortion cancellation for improvedperformance. Moreover, a single design of post-distorter can work withvirtually any RF frequencies. The adaptation is carried out byoptimizing the post-distorter using optimization methods such as randomor gradient optimization routines. The controller 118 is used toregularly read the RBER, provide optimization routines, and properlyprovide for the adjustment of the post-distorter 115. As will be laterdescribed the RBER information is used to adjust control voltages.

[0047] In general terms, the optimization routines can be described asprocedures leading to a goal, i.e., a minimum or in-range value asdefined by an objective or acceptance function. In a gradientoptimization method, values to be optimized are denoted and theoptimization objective is to minimize a squared sum of error functionsdefined by the goal. The gradient optimization calculates the objectivefunctions repeatedly. More specifically, the gradient of the objectivefunction is calculated by perturbing the optimization variables one at atime and finding the corresponding objective function values. Then aline search is performed in the direction of the negative gradient inorder to find a minimum in that direction. At the minimum the gradientis differentiated again and a new search direction is found using thegradient information at the turning point. The iterative process is akinto a root finding algorithm that uses linear approximation in a regionof interest to find roots at points where the approximating line crossesthe axis. In a random optimization method, the optimization variablesare given uniformly distributed values in a min-max range.

[0048] Returning to the system description, FIG. 3 is a block diagramproviding more details of the post-distorter 115. The power splitter 319divides the input IF signal into two paths: a linear path and anonlinear path. In the nonlinear path, the cuber generator 321 generatesdifferential output signals that have the same third-order nonlinearcharacteristics (cuber) of the RF power amplifier. The vector modulator322 provides the amplitude and phase adjustments of the cubic outputsignal. The delay line 320 provides the delay in the linear path that isequal to the total delay of the cuber generator and the vector modulatorin the nonlinear path. The power combiner 323 then combines the linearand the nonlinear signals.

[0049] Next, FIG. 4 is a block diagram providing details of the cubergenerator 321. In the cuber generator, the power divider 424 divides theinput IF signal into two paths. One path goes to the squarer generator427 that generates a squaring output signal. The other path goes todelay line 425 and an amplitude attenuator 426. Each of the two pathsintroduces an associated delay, and the two associated delays are equalto one another. The multiplier 428 multiplies the signals from bothpaths. Then buffer amplifier 429 amplifies and converts the differentialsignals from the multiplier 428 into a single-ended output signal thathas the cuber characteristics.

[0050] As outlined, the cuber generator 321 is configured with a squarergenerator 427. FIG. 5 is a block diagram of the squarer generator. Theinput IF signal goes to the two input ports of the multiplier 530. Thismultiplier multiplies the two signals (x,y) to generate differentialoutput signals that have the squarer characteristics. The bufferamplifier 531 amplifies and converts the differential signals from themultiplier 530 into a single-ended output IF signal. The high passfilter 532 passes the squarer signal and rejects the low frequencysignals resulting from the multiplying process.

[0051] The multiplier 530 used in this embodiment is a monolithic,four-quadrant analog multiplier intended for use in high frequencyapplications with a transconductance bandwidth in excess of very highfrequency (VHF) from either of the differential voltage inputs. Thistype of multiplier is commercially available from differentmanufacturers. Note that the same multiplier is used in the squarergenerator, cuber generator, and vector modulator circuits (427, 321 and322, respectively).

[0052] Other types of multipliers such as high frequency mixer can alsobe used in place of the four-quadrant analog multiplier but then all thecircuit topologies, as described herein, must be changed accordingly.Notwithstanding, the principles of IF post-distorter still apply.

[0053] With respect to the squarer generator circuit 427, consider thetwo sinusoidal input signals v₁ and v₂ as follows:

v ₁ =A cos wt

v ₂ =A cos wt

[0054] where A is the amplitude of the signal and w=2□f, with f beingthe frequency of the signal. The Multiplier provides a total outputvoltage v_(s) given by:

v _(s) =v ₁ *v ₂=(A cos wt)²

v _(s) =A ² cos² wt  (1)

[0055] From a trigonometric function in the form:

Cos 2wt=cos² wt−sin² wt=cos² wt−(1−cos² wt)=2 cos² wt−1

[0056] it follows that:

cos² wt=½(1+cos 2 wt)  (2)

[0057] Then, in view of (2), (1) can be rewritten as follows:

v _(s)=(½A ²)(1+cos 2wt)=(½A ²)+(½A ²)cos 2wt  (3)

[0058] The results in equation (3) contain only the second harmonicfrequency output. Note that no odd harmonic products are present. Theresults contains also a DC term that varies strongly with the amplitudeA of the input signal. Note that (as can be understood from the blockdiagram in FIG. 2) v₁ and v₂ may contain other harmonics. Thus, theabove equations can be expanded further to include the other harmonicsterms.

[0059] With respect to the cuber generator circuit 321, consider thethree sinusoidal input signals v₁, v₂, and v₃ as follows:

v ₁ =A cos wt

v ₂ =A cos wt

v ₃ =A cos wt

[0060] From these input signals, the multiplier provides a total outputsignal v_(s) given by:

v _(s) =v ₁ *v ₂ *v ₃=(A cos wt)³

v _(s) =A ³ cos³ wt  (4)

[0061] From a trigonometric function in the form:

cos 3wt=4 cos³ wt−3 cos wt

[0062] it follows:

cos³ wt=¼(cos 3wt+3 cos wt)  (5)

[0063] In view of (5), (4) can be rewritten as follows:

v _(s)=(¼A ³)(3 cos wt+cos 3wt)

[0064] or:

v _(s)=(¾A ³)cos wt+(¼A ³)cos 3wt  (6)

[0065] Incidentally, since v₁, v₂, and v₃ may contain other harmonics,the equations can be expanded to include other corresponding terms. Notehowever that the results represented in equation (6) contain only thefundamental and third-order products (first and third harmonics with wand 3w, respectively) and no fifth-order products and even-harmonicterms are present. This is in contrast to many pre-distorters intraditional techniques using components such as Schottky diodes thatgenerate fifth-order products and create performance problems in thecommunications system.

[0066] As outlined above, another component in the IF post-distorter 115is the vector modulator 322. FIG. 6 is a block diagram of the vectormodulator 322. As shown, the quadrature hybrid coupler 633 takes thesignal from the cuber generator 321 and provides two output signalsequal in amplitude but 90-degree different in phase. A first multiplier634 multiplies one of these two output signals by a first controlvoltage (control voltage 1) to produce differential output signals whosemagnitudes vary as a function of control voltage 1. The buffer amplifier635 takes these differential signals, amplifies and converts them into asingle-ended output signal. In a similar manner, a second multiplier 636multiplies the other of the two output signals by a second controlvoltage (control voltage 2) to produce another pair of differentialoutput signals whose magnitudes vary as a function of control voltage 2.The buffer amplifier 637 takes these differential outputs signals,amplifies and converts them into a second single-ended output signal.

[0067] As further shown, the power combiner 638 combines the twosingle-ended signals, and produce a summing signal. FIG. 6A illustratesone way in which the RBER is used to provide the control function. Adiagram of the summing signal at the output of the power combiner 638 isprovided in FIG. 7. In this diagram, the signals v₁ and v₂ are twosinusoid signals with a consistent 90 degrees phase shift between themand with respective magnitudes that depend on the control voltage 1 andcontrol voltage 2. Note that it is possible to cover all four quadrantsof the diagram since both control voltages can take either positive ornegative values. Depending on the voltage levels and signs(negative/positive) of the control voltages 1 & 2, the summing signalamplitude and phase can vary from 0 to 30 dB and 0 to 360 degrees,respectively.

[0068] Returning to FIG. 6A, where (in step 602) the RBER is obtainedfrom a performance monitor. The bit error rate performance monitorprovides an inclusive error function for the optimization routine. Theerror function is a single value summarizing distortion from both thetransmitter and receiver. When adjustments utilizing random or gradientoptimization routines are made to control voltage 1 or control voltage2, the result can be determined from the error function. Thus, if theerror function improves, the control voltages are adjusted. If the errorfunction degrades, the control voltages maintain their original values.During optimization when the error function is large (X1, step 604), acoarse tuning procedure is utilized with the control voltages changingin large increments (steps 614 & 616). When the error function is small(X2), a fine tuning procedure is utilized with the control voltageschanging in small increments (steps 606 & 608). After the error functionachieves a predefined value (X2) corresponding to a satisfactorycommunications system performance level, the routine can change to amonitoring loop (steps 610 & 612). In the monitoring loop, if the errorfunction is better than the defined value, then it remains in themonitoring loop, but if the error function degrades below the definedvalue, then it returns to the optimization routine (step 604).

[0069] With these controls, the post-distorter can produce a distortionsignal that has the same magnitude but is out of phase from the combineddistortion signals generated by any transmitter and receiver. Moreover,the adaptive post-distorter continues to optimize the wirelesscommunications system performance regardless of variations intransmitter power levels, or temperature and aging of the transmitterand receiver. The control voltages 1 & 2 come from the controller (118,FIG. 2). The controller, using the inherently available RBER informationfrom the receiver, adaptively adjusts the control voltages 1 & 2 to trimdown the RBER and optimize for the best performance results. Because ofthe relatively easy adjustments of the control voltages 1 & 2, theadaptive process does not need any sophisticated optimization processStandard random or gradient optimization routines are adequate for thisapplication.

[0070] Since all components used in the post-distorter have widebandresponse, the post-distorter has broadband performance coveringdifferent communications applications with any bandwidth (e.g., from 2MHz to 60 MHz) and with any capacity (e.g.,QPSK to 256-QAM). Also,because the post-distorter operates at a common IF frequency, it can beused with any microwave digital communications systems (e.g., from 2 GHzto greater than 40 GHz). Unlike systems with transmitter pre-distorteror feedforward techniques that operate at the high transmitter RFfrequencies, the post-distorter operates at the low receiver IFfrequencies and thus can provide efficient reduction of RBER at a muchlower cost. Moreover, the nonlinear characteristics of the poweramplifiers or receivers do not need to be known in advance of thecancellation process, saving a considerable amount of time in thecalibration process.

[0071] In summary, the present invention provides an adaptive broadbandpost-distortion receiver. In this receiver, the adaptively controlled IFpost-distorter compensates for the non-linear distortion in bothtransmitter and receiver sections of the wireless digital communicationsystem. The post-distorter is placed at the IF section of the receiverand it is configured to cancel the distortion using a bit error ratecalculation.

[0072] Although the present invention has been described in accordancewith the embodiments shown, variations to the embodiments would beapparent to those skilled in the art and those variations would bewithin the scope and spirit of the present invention. Accordingly, it isintended that the specification and embodiments shown be considered asexemplary only, with a true scope of the invention being indicated bythe following claims and equivalents.

What is claimed is:
 1. A system for distortion compensation embodied ina receiver, comprising: a post-distorter; a bit error rate source; and acontroller configured to obtain residual bit error rate (RBER) from thebit error rate source, and use the RBER in performing an optimizationprocess for adjusting the post-distorter to reduce the RBER.
 2. A systemas in claim 1, wherein the post-distorter includes a power splitter, adelay line setting one path from the power splitter, a cuber generatorsetting a second path from the power splitter, a vector modulatorconnected to the cuber generator along the second path, and a powercombiner at which the first and second paths are joined.
 3. A system asin claim 1, wherein the optimization process is a random or gradientoptimization process.
 4. A system as in claim 1, wherein the receiver iscommunicatively connected to a transmitter via an antenna, and whereinthe post-distorter is configured to affect reduction in the RBERregardless of variations in transmitter power levels, or temperature andaging of the transmitter and receiver.
 5. A system as in claim 1,wherein the controller is fashioned as a digital circuit including amicroprocessor.
 6. A system as in claim 2, wherein the cuber generatorincludes a power divider with two outputs each setting a signal path, adelay line on a first one of the signal paths, an attenuator connectedto the delay line along the first signal path, a squarer generator on asecond one of the signal paths, a multiplier configured for receivingsignals from the signal paths, and an amplifier connected to an outputof the multiplier.
 7. A system as in claim 6, wherein the amplifier is adifferential to single-ended amplifier.
 8. A system as in claim 6,wherein the squarer generator includes a corresponding multiplier, ancorresponding amplifier connected to the corresponding multiplier, and afilter coupled to the corresponding amplifier.
 9. A system as in claim8, wherein the corresponding amplifier is a differential to single-endedamplifier.
 10. A system as in claim 8, wherein the filter is a high-passfilter.
 11. A system as in claim 2, wherein the vector modulatorincludes a coupler with two outputs, a first multiplier connected to oneof the coupler's outputs and receiving a first control voltage the valueof which is controlled by the controller, a second multiplier connectedto a second of the coupler's outputs and receiving a second controlvoltage the value of which is controlled by the controller, a firstamplifier connected to the first multiplier, a second amplifierconnected to the second multiplier, and a combiner configured to receivesignals from the first and second amplifiers.
 12. A system as in claim11, wherein the first and second amplifiers are each a differential tosingle-ended amplifier.
 13. A receiver system for distortioncompensation, comprising: means for providing post-distortion; means forproviding a bit error rate; and controller means including means forobtaining residual bit error rates (RBER) from the bit error rateproviding means, and means for using the RBER in performing anoptimization process for adjusting the post-distortion means to reducethe RBER.
 14. A system for wireless digital communications, comprising:a transmitter; and a receiver with a wireless connection to the receiverand with a circuit for adaptive broadband post-distortion, the circuitincluding a post distorter placed at an IF (intermediate frequency)stage of the receiver, a controller, a demodulator, and a basebandprocessor configured for receiving demodulated signal from thedemodulator in response to which the baseband processor provides biterror rate information for the controller, the controller beingconfigured to use the bit error rate information in performing anoptimization procedure for adjusting the post-distorter.
 15. A system asin claim 14, wherein the receiver further includes an RF (radiofrequency) processing and RF to IF converter circuitry configured toproduce an IF signal, and an IF processing circuitry configured toreceive the IF signal and provide a processed IF signal to thepost-distorter.
 16. A system as in claim 14, wherein the post-distorterhas broadband performance and is useable with any microwave digitalcommunication system including one operating in a frequency range from 2GHz to greater than 40 GHz.
 17. A system as in claim 14, wherein thepost-distorter is configured for nonlinear characteristics closelyrelated to those generated by the transmitter and receiver, and reducingfifth-order products.
 18. A system as in claim 14, wherein theoptimization procedure is a random or gradient optimization process. 19.A system as in claim 14, wherein the post-distorter is configured toaffect reduction in the bit error rate regardless of variations intransmitter power levels, or temperature and aging of the transmitterand receiver.
 20. A method for adaptive broadband post-distortion in areceiver system, comprising: receiving an RF signal from a transmitter;converting the RF signal into an IF signal and processing the IF signal;introducing post-distortion into the processed IF signal; demodulatingthe post-distorted, processed IF signal to create a baseband signal;processing the baseband signal; retrieving a bit error rate associatedwith the processed baseband signal; and performing an optimizationprocess according to which the post-distrotion is adjusted.
 21. A methodas in claim 20, wherein the optimization process is a random or gradientoptimization process.
 22. A method as in claim 20, wherein thepost-distortion is configured to affect reduction in the bit error rateregardless of variations in transmitter power levels, or temperature andaging of the transmitter and receiver.
 23. A method as in claim 20,wherein the optimization process involves random or gradientoptimization routines for adjusting first and second control voltages.24. A method as in claim 20, wherein the optimization process includesmonitoring, and adjustment.
 25. A method as in claim 24, wherein themonitoring includes determining if the residual bit error rate (RBER)changes.
 26. A method as in claim 24, wherein the adjustment includesmaintaining control voltage values steady if the RBER degrades,adjusting the control voltage values if the RBER improves, wherein whenthe RBER is larger than a first predetermined value, a coarse tuningprocedure is utilized in which the control voltage values are adjustedin large increments, and wherein when the RBER is smaller than a secondpredetermined value, a fine tuning procedure is utilized in which thecontrol voltages values are adjusted in small increments.
 27. A methodas in claim 26, wherein the monitoring includes entering a monitoringloop when the RBER reaches the second predetermined value, anddetermining if the RBER changes, and existing the monitoring loop toresume the adjustment if the RBER exceeds that value.